Pentostatin and coformycin are tight-binding naturally occurring inhibitors of Adenosine Deaminase (ADA) with a Ki of 2.5×10−12 M and 1.0×10−11 M, respectively (Drug of the future, vol. 15, No. 7, 1990). As expected, Pentostatin greatly enhances the antiviral activity of vidarabine (Ara-A) (Sloan, B. J.; J. K. Kielty & F. A. Miller, Ann. N. Y. Acad. Sci. 284:60-80, 1977.) In later studies, Pentostatin has been found to be effective against chronic lymphocytic leukemia (Kef ford, R. F. & R. M. Fox, Br. J. Haemto. 50: 627-636, 1982; Ho. A. D.; J. Thaler, P. Stryckmans, B. Coiffier, M. Luciani, P. Sonneveld, K. Lechner, S. Rodenhuis, R. Zittoun, J. Natl. Cancer Inst., 82: 1416-1420, 1990) and hairy cell leukemia (O'Dwyer, P. J.; S. Marsoni, M. T. Alonso, & R. E. Wittes, Cancer Treatment Symp. 2: 1-5, 1984; Spiers, A. S. D.; J. C. Ruckdeschel & J. Horton; Scand. J. Haematol, 32: 130-134 1984). Indeed, Pentostatin (Nipent) is now an approved and highly effective agent for the treatment of interferon-refractory hairy cell leukemia (Drug of the future, vol. 15, No. 7, 1990; Cassileth, P. J.; B. Chuvart, A. S. D. Spiers, D E. P. Harrington, F. J. Commings, R. S. Nieman, J. M. Bennett & M. J. O'Connell: J. Clin. Oncol.; 0.9: 243-246, 1991.) Coformycins analogs have attracted more attention in recent years because the inhibitors of AMPDA (Adenosine monophosphate deaminase) may present site- and event-specific drugs that could prevent or attenuate ischemic tissue damage resulting from a stroke or a heart attack (Erion, Mark D.; S. R. Kasibhata, B. C. Bookser, P. D. Van Poelje, M. Rami Reddy, H. E. Gruber & J U. R. Appleman: J. Am. Chem. Soc., 121, 308-319, 1990.)
Two additional analogs with similar activity have been isolated and characterized. They are the 2′-chloro-2′-deoxy compound adechlorine and a carbocyclic analog adecypenol (Tanaka, H.; T. Kawakami, Z. B. Yang, K. Komiyama, S. J. Omura: J. Antibiot., 42, 1722, 1989; Omura, S; H. Tanaka, N. Imamura: J. Antibiot. 39; 309, 1986.)

Due to a wide range of therapeutic applications of coformycins, one can expect demand for the production of these drugs to increase from 500-1000 g/year to multiple kilograms per year.
Pentostatin is currently being produced commercially by large-scale isolation from the fermentation beer of streptomyces antibiticus NRRL 3238 according the procedure that was developed by Showalter-McDonnell-Edmunds at Park-Davis Pharmaceutical, Warner-Lambert Co. (Dion, H. W.; P. W. K. Woo & A. Ryder, Ann. N. Y. Acad. Sci. 284; 21-29, 1977; Woo. P. W. K.; H. W. Dion. S. M. Lang, L. F. Dahl & L. J. Durham, J. Heterocycl. Chem. 11: 641-645, 1974.) The fermentation produces other UV-absorbing contaminations. The major contaminant is the (8S)-isomer (1.1%). 2′-deoxyguanosine is another minor component originally identified in the fermentation beer (Showalter H. D. H., Bunge, R. H., French, J. C., Hurley, T. P., Leeds, R. L., Leya, R., McDonnell, P. D., Edmunds, C. R., J. Antibiotics, 45, 1914-1918, 1992)
Warner-Lambert researchers were also the first to report the complete synthesis of Pentostatin. Their synthetic route is shown in scheme (I).
The earlier synthesis contained total 11 steps and overall yield of 1.6% (Baker, D. C.; S. R. Putt, J. Am. Chem. Soc. 101, 6127-6128, 1979). This involved the synthesis of a five-and seven-membered fused heterocyclic ring aglycone, followed by low-yield glycosylation and reduction of the ketone functionality to produce the 8-R and 8-S isomers. In the following years the Warner-Lambert researchers modified the glycosylation step and obtained 51% of the desired isomer (beta-isomer) and made improvements on other steps. However, the isolation of Pentostatin from the fermentation beer remains to be the only viable preparative method (Chan, E.; S. R. Putt, H. D. H. Showalter, & D. C. Baker, J. Org. Chem. 47: 3457-3464, 1982; Showalter, H. D. H.; S. R. Putt, P. E. Borondy & J. L. Shillis, J. Med. Chem. 26: 1478-1482, 1983).
Other schemes have been reported in the literature. Ohno et al. focus their work on the photo-assisted ring-expansion of nebularine and produced the coformycin in overall yield of 30%. Thier scheme, depicted below, is short and exclusively produces the desired 8-R stereoisomers (Ohno, M.; N. Yagisawa, S. Shibahara, S. Kondo, K. Maeda & H. Umwzawa, J. Am. Chem. Soc. 97, 4326-4327, 1975). However, experiments indicate that the scheme is not applicable to the corresponding deoxy isomer and a much lower yield and extensive decomposition is observed.

A few years later, another method was reported by Rappoport's laboratory at the Berkeley (Truong, T. V. T. & H. Rapoport, J. Org. Chem.: 58, 6090-6096, 1993). As a starting material they used L-vinylglycine which was prepared in three steps from L-methionine methyl ester hydrochloride with a 60% yield (Carrasco, M.; R. J. Jones, S. Kamel, T. Truong, & H. Rapoport; Org. Synth. 70: 29-34, 1991). The exchange of the CBZ protective group with the BOC group was found to be important for separation of syn- and anti-epoxide by flash chromatography (Truong, T. V. T. & H. Rapoport, J. Org. Chem.: 58, 6090-6096, 1993).
As the above scheme shows, using L-vinylglycine as a chiral starting material, the Berkeley researchers have achieved a high yield stereo- and regiospecific synthesis of various protected, enantiomerically pure Pentostatin aglycone. However, none of the intermediates are commercially available and the total 16 synthetic steps make this protocol less desirable for the large-scale production of Pentostatin.
Other synthetic methods reported by Hosmane (Hong, M., Hosmane, R. S., Nucleosides & Nucleotides, 16, 1053-1057, 1997) are represented below:
A synthesis by Montgomery (Thomas, H. J.; J. M. Riordan & J. A. Montgomery, Nucleosides & Nucleotides, 5: 431-439, 1986) is represented below:
These two methods are not desirable for scale-up because they involve either glycosylation with a moderate beta/alpha [β/α] ratio or generation of the C8-hydroxy with the correct stereochemistry at a very late stage of the synthesis, which results in appreciable loss of yield (usually about 40%, see the above scheme).
There are also several reasons for finding an alternative synthetic route to the current enzymatic process of producing Pentostatin.
The major steps used in the original isolation of Pentostatin involved a carbon adsorption/desorption procedure followed by chromatography on Darco G-60 and then on Sephadex G-10. The course of the fractionation was monitored by testing fractions for their ability to inhibit the deamination of adenosine by ADA. Repeated re-crystallization of the product from the final, most active chromatographic fractions afforded less than 8 g of Pentostatin from 9,500 liters of beer. The low yield and considerable labor involved in isolating Pentostatin using this procedure are not practical for the production of kilogram lots of the drug. An alternative process, coupled with a rapid HPLC method for assaying Pentostatin, was later developed. The improved process yielded 648.5 g of >99.7% pure Pentostatin from 50,000 liter of fermentation beer. However, the scale-up difficulties, the number of contaminants and the labor involved remained to be challenging to tackle. Other processes more or less yield the same amounts of the active drug using other fermentation and purification procedures (Kusakabe, H; K. Kodma, H. Machida, Y. Midorikawa, A. Kuninanka & H. Yoshino; Jap. Kokai, 52128292, 1977; JP 5230991, 1977; Omura, S.; H. Tanaka & N. Imamura.: 2′-deoxycoformycin by actinomadura; Jap. 61289896, 1986.).
The enzymatic process, apart from involving considerable labor, suffers from the fact that it still is a biochemical process and several nucleoside contaminations are produced during the fermentation. This poses the challenge to identify and remove them efficiently from the final product. Identified nucleoside contaminations are: coformycin, Ara A, deoxyguanosine, the (8S)-isomer, 8-ketodeoxy- and 8-keto-coformycin (Hanvey, J. C.; E. S. Hawkins, D. C. Baker, R. J. Suhadolink, Biochemistry, 27: 5790-5795 (1988). Separation of these contaminates requires special techniques and expertise. For example a recent patent describes the efficient separation of coformycin, formycin A and isoformycin using CG-50 column and a special elution procedure for separation of coformycin from nucleosides occurring with it (see Fr Pat. 2383966, 1978).
There is a demand for much higher quantities of Pentostatin (Bookser, B. C.; S. Rao Kasibhatla, J. R. Appelman & M. D. Erion, J. Med. Chem. 43: 1495-1507, 2000) and current methods do not comfortably produce such amounts. Thus, a practical, scalable, concise and free from hard-to-separate contaminants that could produce Pentostatin at a competitive price is most desirable at this time. None of the reported synthetic schemes can satisfy all of these requirements.